Ozone has long been recognized as a useful chemical commodity valued particularly for its outstanding oxidative activity. In fact, ozone is the fourth strongest oxidizing chemical known, having an oxidation potential of 2.07 volts. Because of this property, ozone and/or fluid mixtures including ozone are capable of removing a wide variety of contaminants, such as cyanides, phenols, iron, manganese, and detergents, from surfaces. Also, ozone and/or fluid mixtures including ozone are capable of oxidizing surfaces. In particular, ozonated water is used to "clean", i.e., oxidize, the surface of silicon wafers in-process in the semiconductor industry. Additionally, ozone is also useful for inhibiting, reducing and/or eliminating the accumulation of biomass, mold, mildew, algae, fungi, bacterial growth and scale deposits in various aqueous solution systems. When used in this manner, ozonation provides the advantage of producing a lesser quantity of potentially harmful residues than, e.g., chlorination, which leaves undesirable chlorinated residues in aqueous systems.
Because of this wide range of activity, ozone finds application in many diverse processes. Ozone, for example, has been used as a biocide for the treatment of drinking water. Additionally, ozone is used for sterilization in the brewing industry, and for odor control purposes in the sewage treatment industry. Finally, ozonated water finds wide utility in the semiconductor industry, where for example, ozone is used to clean and surface condition in-process silicon wafers. Additionally, as is described in U.S. Pat. No. 5,378,317, ozonated water is used to remove organic materials, such as photoresist, from the surface of silicon wafers. Moreover, ozonated water is used in the semiconductor industry to form a thin, passivating oxide layer on the surface of silicon wafers.
The use of ozonated water provides several advantages in these applications. First of all, because ozonated water is generated at the point of use, it is free of contaminants, i e., particles and metals, that are typically present in chemicals that are stored in barrels or drums. Ozonated water is also less expensive than other oxidizing chemicals and furthermore, since ozonated water naturally decomposes, the use of ozonated water presents no disposal issues. However, the effectiveness of ozone in each of these applications is adversely affected by its low solubility and short-half life (approximately 10 minutes) in aqueous solutions. That is, not only is it difficult to dissolve ozone in an aqueous solution, but also, once dissolved, it is difficult to maintain the ozone in solution.
Although several methods of increasing the quantity of dissolved ozone in aqueous solutions are known, each of these prior art methods has limitations that render them inadequate for certain applications. For example, bubbling ozone directly into water at ambient pressure has been used as a method to dissolve ozone in aqueous solutions. Such a technique, however, does not optimize the quantity of ozone dissolved, since the ozone bubbles effervesce before a substantial amount of ozone can be dissolved into solution and/or before the ozonated water can be applied to the surface to be treated.
Additionally, published European patent application No. EP 0 430 904 A1 discloses a process for producing ozonated water comprising the step of contacting, within a vessel of defined volume, an ozone-containing gas with fine droplets of water. However, this process is less than optimal since it provides limited contact between the ozone-containing gas and water. That is, as the vessel fills with water, the time of contact between the ozone containing gas and the fine water droplets is shortened, resulting in a lesser quantity of ozone being dissolved into solution. Additionally, this application does not teach a method of keeping the ozone in solution until it is delivered to a point of use. Thus, it is possible that, upon delivery, a large quantity of the ozone dissolved in solution will effervesce, and the benefits of the mixing process will be lost.
Finally, several methods utilizing cooling to increase the quantity of dissolved ozone in aqueous solutions have also been proposed. For example, U.S. Pat. No. 5,186,841 discloses a method of ozonating water comprising injecting ozone through an aqueous stream across a pressure drop of at least 35 psi. The ozonated stream is then combined with a second stream that is preferably a portion of an aqueous solution which is recirculating in a cooling water system. The resultant stream is forced to flow at a velocity of 7 feet per second for a distance sufficient to allow 70% of the ozone to be absorbed. Additionally, U.S. Pat. No. 4,172,786 discloses a process for increasing the quantity of dissolved ozone in an aqueous solution by injecting an ozone containing gas into a side stream conduit which circulates a portion of cooling water. The ozone-injected water is then mixed with the cooling water in a tower basin, thereby ozonating the water. Finally, U.S. Pat. No. 5,464,480 discloses a process for removing organic materials from semiconductor wafers using ozonated water. Specifically, this patent teaches that high ozone concentration water, suitable for use in the disclosed process may be obtained by mixing ozone and water at a temperature of from about 1.degree. C. to 15.degree. C.
Although the systems disclosed in U.S. Pat. Nos. 5,186,841, 4,172,786 and 5,464,480 claim to increase the quantity of dissolved ozone in water, it is more likely that much of the ozone effervesces to the atmosphere and/or is converted to oxygen rather than being dissolved in the water. Thus, these systems would require the use of a large amount of ozone, which would, in turn, render them costly. Additionally, these patents do not disclose methods for optimizing the ozone concentration at the point of use, and as a result, it is possible that the increased ozone, if any, that is dissolved as a result of cooling the solution, will effervesce out of solution at the point of use.
Thus, there is a need for an efficient method of increasing the quantity of ozone that may be dissolved and maintained in aqueous solution to a point of use, not only to minimize the amount of ozone used, but also to provide sufficiently ozonated aqueous solutions for given applications.